Electromagnetic couplers, such as radio frequency (RF) couplers are used in a variety of applications to extract a signal for measurement, monitoring, or other uses. For example, an RF coupler can be included in a signal path between an RF source and a load (such as an antenna) to provide an indication of forward RF power of the RF signal traveling from the RF source to the load and/or an indication of reverse RF power reflected back from the load. An RF coupler typically has a power input port, a power output port, a coupled port, and an isolated port. When a termination impedance is presented to the isolated port, an indication of forward RF power traveling from the power input port to the power output port is provided at the coupled port. When a termination impedance is presented to the coupled port, an indication of reverse RF power traveling from the power output port to the power input port is provided at the isolated port. The termination impedance is typically implemented by a 50 Ohm shunt resistor in a variety of conventional RF couplers.
FIG. 1 is a block diagram illustrating an example of a typical arrangement of an RF “front-end” sub-system 10 as may be used in a communications device, such as a mobile phone, for example, to transmit RF signals. A power amplifier 11 provides gain to an RF signal input to the sub-system, producing an amplified RF signal. A filter 12 is used to filter out unwanted frequencies from the amplified RF signal. An RF coupler 13 is used to extract a portion of the power from the RF signal traveling between the filter 12 and an antenna 14. The antenna 14 transmits the RF signal.